The exhaust gas from a spark ignition or diesel engine which is operated on a hydrocarbon fuel will typically contain small amounts of carbon monoxide, organic compounds (such as hydrocarbons), nitrogen oxides and sulfur oxides which are undesirable emissions. The carbon monoxide and organic compounds result from incomplete combustion of the fuel, the nitrogen oxides are primarily a result of the combination of nitrogen and oxygen from the air which is used to burn the fuel in the engine, and the sulfur oxides result primarily from sulfur which is present as an impurity in the fuel. Small amounts of sulfur oxides can also result from sulfur compounds that may be present as components of the engine's lubricating oil.
Emission control systems for vehicles which are powered by an internal combustion engine typically include a catalytic converter which is used to catalyze the conversion of harmful emissions in the engine exhaust to gases which are less objectionable. Catalysts have been developed for use in catalytic converters which are highly effective in catalyzing: (1) the oxidation of organic compounds to water and carbon dioxide; (2) the oxidation of carbon monoxide to carbon dioxide; and (3) the conversion of nitrogen oxides to less objectionable products. However, the performance of such catalysts is often adversely affected by sulfur compounds which can act as catalyst poisons.
Sulfur oxide emissions can be controlled by reducing the amount of sulfur which is contained in the fuel. This approach will be used in California where the California Air Resources Board has established Phase 2 specifications for reformulated gasoline which become effective in March of 1996. These Phase 2 specifications define a maximum sulfur content for gasoline of 40 ppm as a flat limit for producers whereas, previously, conventional gasoline in California could contain up to 150 ppm of sulfur. Unfortunately, this approach is ultimately limited by the high cost of producing fuels which have a very low sulfur content.
It has been recognized for many years that the performance of a catalyst which is used to promote the destruction of hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas from an engine can be improved if the catalyst is protected from catalyst poisons that can be present in the exhaust gas. For example, U.S. Pat. Nos. 3,44,886 and 3,429,656, both to Taylor et al. teach the use of a preconditioning zone containing a guard material which chemically reacts with and removes exhaust gas components, such as sulfur oxides, which would otherwise deactivate the catalyst. It is disclosed that a suitable guard material will contain a combination of 45 to 90 parts by weight of calcium as CaO, 5 to 30 parts by weight of SiO.sub.2 and 5 to 25 parts by weight of sodium as Na.sub.2 O. It is further disclosed in the Taylor et al.--656 patent that the CaO--Na.sub.2 O--SiO.sub.2 guard materials can be made more effective when used in association with oxides of metals from the group consisting of Cu, Mn, V, Cr, Fe, Co, Ni and Mo.
British Patent Specification No. 1,444,444 discloses that when a catalyst is used to promote the afterburning of exhaust gases from an engine, lead compounds and sulfur in the fuel and also phosphorous and zinc in the engine lubricating oil have an adverse affect on the catalyst performance. It is further disclosed that catalyst poisons in the exhaust gases can be removed by passing the exhaust gases through an absorptive material for such poisons prior to passage through the catalyst. The absorptive material is preferably formed of porous alumina pellets which are capable of absorbing lead compounds in the exhaust gases. It is disclosed that sulphur or phosphorous can also be absorbed by adding a substance which is reactive with sulfur or phosphorous.
Published European Patent Application No. 0 582 917 A1 (Goto et al.) is directed to an exhaust gas purification device for an engine wherein an NO.sub.x absorbent is used to absorb NO.sub.x emissions in the exhaust gas. It is disclosed that sulfur oxides in the exhaust gas have an adverse effect on the ability of the NO.sub.x absorbent to absorb NO.sub.x. It is further disclosed that a sulphur trapping absorbent can be placed in the exhaust gas upstream of the NO.sub.x absorbent for the purpose of preventing sulfur oxides from flowing into the NO.sub.x absorbent. The SO.sub.x absorbent contains at least one substance selected from alkali metals, alkali-earth metals, rare-earth metals, and precious metals such as platinum. Alumina can be used as a carrier for the SO.sub.x absorbent.
Unfortunately, modern catalytic converters only operate after reaching temperatures in excess of about 300.degree. C. For this reason, a substantial portion of hydrocarbon emissions from an internal combustion engine usually occur during the first few minutes of cold-start engine operation before the converter reaches its minimum effective operating temperature. This minimum effective operating temperature is frequently referred to as the converter "light-off" temperature. Because the first few minutes of operation are an integral part of automotive emissions tests, and because over 60% of the measured hydrocarbons can be emitted during the cold-start period of the test, a reduction of cold-start hydrocarbon emissions is of critical importance. Recent tightening of emissions requirements to limit emissions of certain hydrocarbon compounds, such as benzene, has further underscored the need for reduced cold-start hydrocarbon emissions.
The use of a sulfur absorbent to remove sulfur oxides from an exhaust gas before they can reach and poison a catalytic converter is an effective way to protect and maintain the activity and lifetime of the catalytic converter which is used to catalyze the conversion of carbon monoxide, organic compounds, and nitrogen oxides to less objectionable products. Although the use of such an absorbent solves one problem, it also creates an entirely new problem. A large quantity of absorbent must be used in view of the fact that it must have the ability to absorb significant quantities of sulfur oxides. By way of illustration, if a gasoline containing 150 ppm of sulfur and having a density of 6.5 pounds per gallon (0.7789 g/cm.sup.3) is used as the fuel for an automobile that has an overall fuel economy of 20 miles per gallon (8.50 km/l), a total of 0.488 pounds (221 g) of sulfur will be discharged in the exhaust gas for every 10,000 miles (16,100 km) of operation. Accordingly, a significant mass of absorbent will be required to capture this sulfur. Because of its mass, the absorbent will absorb a significant amount of heat from the exhaust gases upon cold-start of the engine, and the presence of this heat sink will increase the amount of time required before the catalytic converter reaches its "light-off" temperature. Accordingly, the use of a sulfur oxide absorbent to protect the catalytic converter in accordance with the teaching of the prior art will cause an undesirable increase in cold-start hydrocarbon emissions.
The problem of cold-start hydrocarbon emissions has been addressed through the use of adsorbents which have the ability to adsorb hydrocarbons from the exhaust gas at low temperatures and then release the adsorbed hydrocarbons at higher temperatures when the catalytic converter has reached its "light-off" temperature. For example, U.S. Pat. No. 5,158,753 (Take et al.) and U.S. Pat. No. 5,303,547 (Mieville et al.) disclose processes wherein an adsorbent is used to adsorb organic substances from the exhaust gas in combination with a heat exchanger which transfers heat from the exhaust gas to the catalytic converter by means of a heat exchanger before the exhaust gas is, passed through the adsorbent. At the low temperatures of a cold-start, the hydrocarbon adsorbent adsorbs hydrocarbons from the exhaust gas. As the hydrocarbon adsorbent is heated to high temperatures by continued exposure to the exhaust gas, the adsorbed hydrocarbons are desorbed and then passed to the catalytic converter which has been heated by indirect heat exchange.
Published International Patent Application No. WO 94/11623 (Burk et al.) is also directed to the use of a hydrocarbon adsorbent to reduce cold-start hydrocarbon emissions from an engine. More specifically, it discloses the treatment of an engine exhaust gas through the use of a first and a second catalyst zone and a hydrocarbon adsorbent zone between them, wherein the first and second catalyst zones are in heat transfer relation to one another. Heat transfer from the first catalyst zone to the second catalyst zone helps to bring the second catalyst therein more quickly to its effective operating temperature, and the hydrocarbon adsorbent in the adsorbent zone reduces the quantity of hydrocarbons discharged to the atmosphere during engine cold-start by adsorbing hydrocarbons from the exhaust gas until the second catalyst reaches a temperature at which it can more effectively convert the hydrocarbons to innocuous substances. This publication also teaches that air can be added to the exhaust gas stream at a point upstream of the second catalyst zone.